Process and plant for obtaining steel strip coils having cold-rolled characteristics and directly obtained in a hot-rolling line

ABSTRACT

A process for obtaining steel strip coils with characteristics of a cold-rolled product, directly in a hot-rolling line, comprises subsequently to steps of casting and thickness reduction at a temperature of more than 1100° C. upon solidification, induction heating of the product and a further step of hot rolling, above point Ar 3 , a step of cooling and temperature control in a range of between 600° and 250° C., thus lower than said point Ar 3 , as well as one or more passes of cold-rolling in series, with final coiling of the obtained product. Also a preferred plant is described for putting into practice such a process.

The present invention relates to a process and relevant plant forproducing coils of steel strips, having characteristics of a cold-rolledproduct and directly obtained in a hot-rolling line from a continuouscasting with arc-shaped path and horizontal outlet.

It is known that for obtaining hot-rolled steel strip coils, thefollowing operations are provided, successively:

producing by casting a steel slab having a thickness of between 160 and250 mm, and possibly storing the same;

heating such a slab, if coming from the store, or in any case bringingit again to a rolling temperature of at least 1050° C.;

hot-rolling the slab for a first cogging and thereafter for obtainedhot-rolled strips having a minimum thickness of 2 mm;

taking again the hot-rolled strip and subjecting the same to annealingfor a reconstruction of the grain which has been deformed and becomedishomogeneous during the preceding operations, in particularhot-rolling step;

subjecting the product to pickling in order to eliminate from itssurface the oxides previously formed, especially during annealing; and

causing the actual cold-rolling step to be performed, which comprisesmounting the coil onto an unwinding reel to bring again the strip onto aplane, causing the strip to pass through at least one cold-rolling standuntil obtaining thicknesses of less than 1 mm, down to 0.5-0.2 mm andfinally winding the strip on a reel to obtain the final coil.

It will be noted that the number of passes in the stands forcold-rolling depends on the desired final thickness and the reductionpercentage which is to be obtained, in other words the ratio betweenthickness of the hot-rolled strip and thickness of the final product.For high values of such a percentage reduction it is not enough toincrease the number of said passes, but it will be necessary to subjectthe strip to another annealing operation and the consequent pickling,otherwise the material hardens and the final product results to be oflow quality.

Although it is possible to obtain by hot-rolling strips having athickness of less than 2 mm, hot-rolling is usually avoided to reachthese values, as this type of processing is considered anti-economical,above all due to the reduced productivity that would be obtained in thiscase with a conventional rolling mill. The costs relating to thereduction of strip thickness are however extremely high in any case.Assuming 100 the cost of hot-rolling, starting from liquid steel, thecost of cold-rolling step alone is of at least 80.

Attempts have been described as to making plants for obtaining thinstrips by means of more compact operating cycles with respect to theabove-mentioned conventional cycle, in order to reduce complexity andduration of the latter. For example EP-A-226446 describes a number ofhot-rolling examples, all in line and at a very high speed (not lessthan 1500 mm/min) but the final product not only has a thickness of 2-6mm, falling thereby in the range of hot-rolling, but also certainly doesnot show the structural features of a cold-rolled product. The mainpurpose of this published application is in fact restricted to a highproductivity while obtaining at the same time a product of goodprocessability, but not of high quality.

In EP-370 575 there is described a method for the manufacture of a steelstrip having a final thickness of between 0.5 and 1.5 mm, comprising thesteps of hot-rolling a steel slab of less than 100 mm thickness, at atemperature of between 300° C. and a temperature at which at least 75%of the material is converted into ferrite, with a thickness reduction ofover 30% in at least one reduction stage, and an exit speed afterhot-rolling of less than 1000 m/min, with final coiling of the stripafter recrystallization. This was an attempt to avoid the two successivecycles of hot- and cold-rolling, with intermediate stages of annealingand pickling, but also this attempt has been unsuccessful, and it couldnot find success indeed, apart from the proposed solution, as in anycase the inner structure of the material, when subjected tocold-rolling, is unsuitable to undergo this treatment for obtaining afinal product of acceptable quality. This occurs owing to the fact thatthe inner structure, if not recrystallized before cold-rolling, resultsto be dishomogenous under a dimensional aspect and with insufficientlyfine grains, in comparison with the grain size which would be requiredby the conventional cold-rolling technology according to theabove-described cycle.

It is known on the other hand that an eccessive reduction of thicknesswith successive rolling stands on the same hot-rolling line gives riseto such a temperature decrease to go below the recrystallization pointAr₃, at which the steel is no longer austenitic, whereby a subsequentannealing above Ar₃ restores the pre-existing structural situationwithout the benefits of grain reduction.

An attempt in improving the results obtained is disclosed in EP-A-0 306076 according to which a first rolling step of the product in theaustenitic region is followed by a subsequent rolling step in theferritic region, with the two steps being separated by means of anintermediate cooling step. However, in order to produce high qualitysteel, recrystallization annealing and possibly pickling steps arerequired.

Instead it has been surprisingly found that if a preliminary reductionof the thickness is carried out in a liquid core situation of thecasting product immediately under the mould, followed by a furtherreduction of thickness at temperatures higher than 1100° C., the productentering the second rolling step shows an inner structure with finegrains, so uniformly distributed to have the characteristics of amaterial suitable to be cold-rolled. Therefore it has been thought thatrolling up to thickness of less than 1 mm can be obtained with no needof annealing and pickling, as it can be in practice performed in linewith the hot-rolling carried out upstream.

In this way a technical prejudice can be overcome, which is extremelycommon and deep-rooted both among those skilled in hot-rolling, andthose, normally distinct therefrom, who are skilled in cold-rolling,since the material obtained in the hot-rolling line results to besuitable to cold-rolling, even if its temperature is caused to be lowerthan recrystallization point Ar₃.

It was found to be important to reach such results, that during thefirst stage of hot-rolling in the austenitic region the temperature iskept as homogeneous as possible at about 1100° C. by inductionre-heating, as disclosed in WO 89/11363.

Therefore it is an object of the present invention to provide a processand relevant plant for obtaining a cold-rolled product, of extremelythin thickness, directly starting from the hot-rolled product, beingcoupled thereto as to the speed and with no need of further treatment(such as annealing and pickling) on the material, thereby without anydiscontinuity in the manufacturing line.

This is obtained by means of a process according to the features ofpresent claim 1.

It should be appreciated that the expected temperature at the outletfrom the controlled cooling device is always less than that ofrecrystallization point Ar₃, which varies according to the carboncontent in the steel, with a minimum of 690° C. for 0.6% of carbon, upto a maximum of 900° C. for lower or higher carbon contents. Thereforeit is certain that the subsequent processing is actually a cold-rollingstep, which is carried out on a material the inner structure of whichhas all the required characteristics in order that the cold-rollingoperation is accomplished in the best way and the final product isprovided, from a metallurgical point of view, with all the propertieswhich are required to a cold-rolled product.

These and further objects, advantages and features of the processaccording to the present invention, as well as of the relevant plant,will be clear to those skilled in the art from the following detaileddescription of a preferred embodiment, given by way of a non-limitingexample with reference to the annexed drawing showing a diagrammaticview of a plant according to the invention, useful to describe also theprocess of the invention.

From a continuous casting mould 10, the steel flat product 1, driven andguided by a known-type roller path being arc-shaped, from an initiallyvertical direction, passes through the arc-shaped path formed by therollers 11, to a horizontal direction. The thickness of the castingproduct 1 is firstly reduced in a condition of liquid core, for examplein two distinct sections of rollers 13 and thereafter, uponsolidification, but still at a temperature of about 1100° C., in a firststage of rolling 15 at the end of the bent path 11 and at the beginningof the horizontal path. Subsequently, in an induction oven 21 the flatproduct 1 is re-heated to bring it again to hot-rolling temperature, andthen rolled in one or more rolling stands 27, between which there may bepossibly provided additional induction ovens (not shown in the drawing)for maintaining the rolling temperature of at least 865° C. at theoutlet from the stand.

According to an embodiment of this first portion of the plant,substantially already known from WO 89/11363, immediately after thefirst rolling stage 15 there may be provided a shear 17 and before saidstage 15 a discaling device 19 for eliminating scale from the surface ofthe product to be treated. Furthermore, between the induction oven 21and the hot-rolling stands 27 there may be provided a winding andunwinding device 23 comprising a reel 22 for coiling the strip from oven21, being coupled to a reel 24 for uncoiling the strip itself to be fedto stands 27, possibly after an additional discaling step in a suitabledevice 25 provided at the inlet of the first rolling stand.

According to the present invention, the hot-rolled strip 1, at theoutlet of the last rolling stand 27 at a temperature certainly higherthan the recrystallization point Ar₃, is caused to enter, still in thesame production line, a cooling and temperature controlling apparatus29, at the exit of which the strip has a temperature, controllable ateach time, comprised in a range of between 250° and 600° C. Itsubstantially consists of a waterbased cooling device, for example ofthe so-called "laminar rain" type, being provided with a temperaturedetector with a feed-back controlling the valves for feeding water intothe device. The value of temperature to be fixed for strip 1 at theinlet of the subsequent cold-rolling stage, with deviations of not morethan 20° C., will depend on the type of steel (carbon content, etc.),the feeding speed of the strip and its thickness, but in any case itwill be less than the temperature at the recrystallization point Ar₃,which varies between 900° C. and a minimum of 690° C. for a carboncontent of 0.6%. As the maximum temperature provided at the outlet ofapparatus 29, thereby at the inlet of subsequent cold-rolling stage 31,is of 600° C., the strip is surely under the point Ar₃, and thus at thebest conditions to undergo the cold-rolling step, due to the fine grainstructure of the material from the upstream treatment, absolutelysuitable to be subjected to cold-rolling.

Such a rolling occurs in at least one stand, for example of "six high"type, i.e. with six rolls mounted in vertical. The passes ofcold-rolling may however be more than one, but all in series whenproviding a multiplicity of stands side by side, contrary to the methodof providing for a multiplicity of subsequent passes in the same stands,as according to the conventional technology of cold-rolling.

Finally the cold-rolled strip, with a thickness of less than 1 mm, readyfor use as it shows the tipical microcrystalline features of acold-rolled product, such as a homogeneous distribution of grains, iswound on a final coiler 33. The lower limit of the thickness that can beobtained in this way will be only dictated by the nip of thecold-rolling stands 31, as well as their precision, not certainly byproblems of material hardening or anyhow deriving from its metallurgicalstructure.

We claim:
 1. A process for obtaining steel strip coil directly from ahot-rolling line, said steel strip coil having characteristics of acold-rolled product, comprising:mould casting a flat product having athickness of less than 100 mm; reducing the thickness of said flatproduct immediately beyond the mould while maintaining a liquid core ofsaid flat product; after solidification of the core of said flatproduct, further reducing the thickness of said flat product by rollingat a temperature greater than 1100° C. to provide a flat product of10-30 mm thickness; induction re-heating said flat product to provide asubstantially homogeneous temperature throughout said flat product ofabout 1100° C.; subjecting said flat product to a further stage ofhot-rolling in the austenitic region at a temperature above atemperature value corresponding to transformation point Ar₃ ; reducingthe temperature of the flat product to a temperature lower than atemperature corresponding to said transformation point Ar₃ ; andsubjecting said flat product to at least one cold-rolling to obtain saidsteel strip having characteristics of cold-rolled product.
 2. A processaccording to claim 1, wherein said hot-rolling stage at a temperaturebelow the transformation point Ar₃ is carried out in the range of 250°C.-600° C.
 3. A process according to claim 1, further comprising atleast one of the following steps:(i) coiling and subsequent uncoiling ofthe strip immediately after the induction re-heating, upon cutting thestrip immediately after the first rolling stage; (ii) at least onediscaling step; and (iii) additional heating between two further stagesof hot-rolling.
 4. A plant for obtaining steel strip coils havingcharacteristics of cold-rolled product, directly obtained in ahot-rolling line, comprising:means (10) for continuous casting of flatproduct (1) with a subsequent arc-shaped guide roller path (11); firstreduction means (13, 15) for reducing thickness of the flat product insaid arc-shaped path when it is in a condition of liquid core orimmediately thereafter upon solidification of said flat product (1);heating means (21) for induction heating and homogenization oftemperature throughout of the flat product (1) downstream of said firstreduction means; at least one additional rolling stand (27) downstreamfrom said heating means; cooling means (29) for cooling and controllingthe temperature of the flat product (1) until under transformation pointAr₃ immediately downstream of said additional hot-rolling stand (27); atleast one cold-rolling stand (31) downstream of said cooling means; andcoiler means (33) for winding the strip in a coil (1') downstream ofsaid cold-rolling stand.
 5. A plant according to claim 4, furthercomprising at least an additional induction oven intermediate betweentwo subsequent ones of said additional rolling stands (27).
 6. A plantaccording to claim 4, characterized in that the range of variation ofthe temperature at the exit of said device (29) is between 250° and 600°C., with a deviation of more or less 10° from the prefixed value at theinside of said range according to the quality of steel, the feedingspeed and the product (1) thickness.
 7. A plant according to claim 6,further comprising a device (23) for winding and subsequently unwindingthe strip immediately downstream of the induction oven (21), upstream ofthe latter being provided a shear cutting device (17).
 8. A plantaccording to claim 7, further comprising at least a discaling device(19, 25), respectively upstream of the first rolling stage (15) anddownstream of said induction oven.
 9. A plant according to claim 4,wherein said cooling apparatus (29) is a water cooling device with waterfeeding valves and having a temperature detector with feed-back forautomatic controlling the cooling water feeding valves.
 10. A plantaccording to claim 9, further comprising a device (23) for winding andsubsequently unwinding the strip immediately downstream of the inductionoven (21), upstream of the latter being provided a shear cutting device(17).
 11. A plant according to claim 9, further comprising at least adiscaling device (19, 25), respectively upstream of the first rollingstage (15) and downstream of said induction oven.
 12. A plant accordingto claim 9, further comprising at least an additional induction ovenintermediate between two subsequent rolling stands (27).
 13. A plantaccording to claim 9, wherein said cooling means (29) comprises meansfor cooling the flat product when leaving said cooling means to withinthe range of 250° to 600° C., according to the quality of steel, thefeeding speed and the flat product (1) thickness.
 14. A plant accordingto claim 13, further comprising at least a discaling device (19, 25),respectively upstream of the first rolling stage (15) and downstream ofsaid induction oven.
 15. A plant according to claim 13, furthercomprising at least an additional induction oven intermediate betweentwo subsequent rolling stands (27).
 16. A plant according to claim 4,further comprising means (23) for winding and subsequently unwinding theflat product immediately downstream of the heating means (21), upstreamof the heating means there being provided a shear cutting device (17).17. A plant according to claim 16, further comprising at least anadditional induction oven intermediate between two subsequent rollingstands (27).
 18. A plant according to claim 16, further comprisingdiscaling means upstream of the first reduction means (15) or downstreamof said heating means 21, or a discaling means (19, 25) both upstream ofsaid first reduction means and downstream of said heating means.
 19. Aplant according to claim 18, wherein the discaling means (25) downstreamof the heating means (21) is positioned immediately after said windingand unwinding means (23).